[slimtensor] Add from_blob() factory function for wrapping external memory

[Gasoonjia]

Stack from ghstack (oldest at bottom):

• [slimtensor] integration into backend #16565
• [slimtensor] Add from_etensor factory function for ETensor to SlimTensor conversion #16551
• rename slimtensor fiies to consistent with executorch files #16469
• [slimtensor] Add utility functions to common_shims_slim #16457
• [slimtensor] Add storage and device property getters to common_shims_slim #16455
• [slimtensor] Add common_shims_slim with basic property getters #16454
• [slimtensor] Add aoti_torch_item_bool for SlimTensor #16453
• [slimtensor] Add aoti_torch_assign_tensors_out for SlimTensor #16452
• [slimtensor] Add aoti_torch_copy_ for SlimTensor #16451
• [slimtensor] Add aoti_torch__reinterpret_tensor for SlimTensor #16450
• [slimtensor] Add aoti_torch_new_tensor_handle for SlimTensor #16449
• [slimtensor] Add aoti_torch_delete_tensor_object for SlimTensor #16448
• [slimtensor] Add SlimTensor-based aoti_torch_empty_strided #16447
• [slimtensor] Add SlimTensor-based aoti_torch_create_tensor_from_blob_v2 #16446
• [slimtensor] Introduce CUDA guard to aoti/slim/cuda #16724
• [slimtensor] Add item() methods with CUDA and CPU support for scalar value extraction #16445
• [slimtensor] Add permute() and reshape() view operations #16444
• [slimtensor] Add as_strided() view operation for tensor reinterpretation #16443
• -> [slimtensor] Add from_blob() factory function for wrapping external memory #16442

Add from_blob() factory function that creates SlimTensor from external
memory without taking ownership. This enables creating tensors that wrap
user-provided buffers for use in AOTI model execution.

Changes:

• Create factory/FromBlob.h with from_blob() overloads
• Support sizes/strides as ArrayRef or initializer_list
• Support custom storage offset
• Add from_blob target to factory/targets.bzl

Differential Revision: D89950482

[pytorch-bot]

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❌ 2 New Failures

As of commit 1458797 with merge base c7c4073 ():

NEW FAILURES - The following jobs have failed:

• pull / test-coreml-bc-macos (macos-m2-stable) / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1
• Test CUDA Builds / test-model-cuda-e2e (mistralai, Voxtral-Mini-3B-2507, quantized-int4-tile-packed) / linux-job (gh)
  RuntimeError: Command docker exec -t 7331fa06ab4763b83a76090c3279f2659791da32e86d01b71206ce34977bb51e /exec failed with exit code 1

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